The circadian clock

2. Introduction

2.1 The circadian clock

During evolution, all living organisms must adapt to variations in environmental conditions.

The most striking changes on earth are the alternation between day and night, the seasonal variations of their relative durations, and the temperature fluctuations associated with them.

These changes are the result of the cyclic geophysical properties of our planet, that is, it rotates around the sun in 365 days and rotates around its own axis in 24 hours with an angle of 23°27’.

Most eukaryotes and some prokaryotes are not only able to respond to these changes, but have developed an internal timing system allowing them to measure time, anticipate the environmental daily changes, and tune their physiology and behavior to these alterations. This system is based on molecular cyclic events running with a period length (τ or tau) of approximately 24 hours and is named the circadian clock (circadian been derived from the Latin words circa : about and diem : day). Because the circadian period produced by this auto regulatory mechanism is not exactly 24 hours, this timekeeper needs to be reset regularly.

This is achieved every day by the photoperiod, which also allows an adaptation to seasonal

changes. In turn, the circadian clock drives the different biological processes occurring in the organism in order to synchronize them with geophysical time in the most favorable way.

Light is the most important Zeitgeber (German for “time giver”). Nevertheless various other external stimuli such as temperature, nutrients availability, social interactions, etc. are able to reset the phase of the circadian clock. In nocturnal animals such as mice, light pulses given during the night are able to phase shifts the circadian system, while if they are given during the day, they have no effect. Moreover, if they are applied during the fist half of the night, they provoke phase delays, while if applied during the second half of the night, they engender phase advances (Daan and Pittendrigh, 1976). In a similar manner, temperature changes can phase-shift the circadian rhythm of locomotor activity and of eclosion in Drosophila (Zimmerman et al., 1968).

However, while temperature variations can reset the phase of the circadian clock, long-term high or low temperatures have nearly no influence on the period length of the oscillator. This astonishing property, called temperature compensation, allows the oscillator to continue to run with a relatively stable period length in a wide range of temperatures (Izumo et al., 2003; Tsuchiya et al., 2003). This is in sharp contrast with the strong temperature dependence observed for most biochemical reactions. This feature is very relevant for time keeping in cold-blooded organisms whose body temperature can be subject to important daily or seasonal changes.

The circadian clock is self-sustained, that is, it continues to run in constant conditions. This was first shown in 1729 by the French astronomer Jean-Jacques d'Ortous De Mairan. He demonstrated that the sunlight was not necessary for the movements of mimosa leaves by placing the plant in total darkness. Thus, even under these nearly constant conditions, the leaves opened during the subjective day and folded during the subjective night (DeMairan, 1729) (Figure 1). The existence of an internal clock was then shown to be present also in animals, including humans. In 1965, a study of Jurgen Aschoff and Rutger Wever revealed

the persistence of a close to 24 hours rhythmicity of many aspects of behavior and physiology in voluntary subjects retreated into a bunker devoid of environmental cues (Aschoff, 1965).

The presence of circadian rhythms in some prokaryotes, such as Synecocchus elongatus cyanobacteria, was the first evidence that a molecular oscillator can reside within a single cell (Golden et al., 1997). In the context of multicellular organisms, each cell represents an autonomous oscillator that does not require intercellular communication to create the oscillation. However interactions between cells can play an important role in the

synchronization of individual clocks at the level of a tissue. Indeed, autocrine or paracrine secretion can synchronize and/or reinforce cell autonomous oscillations (Harmar et al., 2002;

Liu et al., 2007; Peng et al., 2003). In addition, the coordination of multiple oscillators throughout a whole organism can be achieved owing to the establishment of a hierarchical organization of these clocks. In mammals for example, specialized groups of neurons that constitute the suprachiasmatic nuclei (SCN) in the anterior hypothalamus coordinate the oscillators present in the cells of adjacent brain regions and of the different peripheral organs Figure 1. A representation of de Mairan’s original experiment.

When exposed to sunlight during the day (upper left), the leaves of the plant were open, and during the night (upper right) the leaves were folded. De Mairan showed that sunlight was not necessary for the leaves movements by placing the plant in total darkness: even under these constant conditions, the leaves opened during the day (lower left) and folded during the night (lower right). This figure is taken from (Moore-Ede et al., 1982).

via neural and/or humoral pathways (Reppert and Weaver, 2001). This “central clock” is located just above the optic chiasm and is synchronized by visual input from the photoperiod via direct and indirect pathways from the retina. In turn, it generates coordinated circadian outputs that regulate overt rhythms.

The ubiquitous presence of the circadian mechanism in organisms from cyanobacteria to green plants and humans implies that it fulfills functions that provide selective advantages to the organism. In all examined organisms circadian clocks are dispensable under laboratory conditions. However, under natural conditions, which are much more hostile, circadian clock must increase survival fitness. It should be emphasized that features providing only a small increase in survival per generation will eventually be selected after many generations. An increase in fitness provided by circadian clocks has been beautifully exemplified in the laboratory for cyanobacteria (Synecocchus elongatus) and the plant Arabidopsis Thaliana by competition experiments. In cyclic environmental conditions, a Synecocchus strain with a functioning clock rapidly outcompetes a clock-defective strain. However this selective advantage disappears in constant environment. In addition, competition experiments between strains with different period lengths revealed that a strain competes more effectively if its period length resonates with that of an imposed light-dark cycle (T-cycle) (Woelfle et al., 2004). Similarly, Dodd and co-workers have shown in Arabidopsis Thaliana that plants with a period length that matches that of the environmental rhythm contain more chlorophyll in their leaves, fix more carbon, grow faster, and survive better than plants with circadian periods differing those of environmental cycles (Dodd et al., 2005).